This invention is directed to the preparation of a lining for metal containers such as standard steel drums as normally used for packing, transporting and storing materials over relatively long periods of time. Corrosive foodstuffs when packed in drums must be maintained in an aseptic condition so that contamination from defects in the protective lining of the drum occurring by reaction between the metal and the contents of the container is avoided. Typical contents are tomato paste, olives, cherries, wine concentrate and other acidic substances which form undesirable reactions when in direct contact with the metal containers. Of the above, tomato paste presents the most serious corrosion problem.
A typical metal container as employed with this invention is a so called "oil drum" type of container which is normally a 55-gallon cylindrical container used for industrial purposes such as transport of petroleum products, chemicals and many other products where the likelihood of corrosion is not as great as in the case of tomato paste and other corrosive materials, or where the consequences of such corrosion and resultant contamination are not as serious.
Apart from the present invention, there are only two commercial systems for packaging tomato paste corrosive foodstuffs in steel or other metal drums. One of them uses a product wherein at least all of the interior area of the drum, that is, the shell, top and bottom lids, are tin-coated and then coated with a phenolic resin. Tin-coating systems are very expensive and require high capital expense. Moreover, tinplate is a very expensive material to use for this purpose. The other commercial type of drum as used for tomato paste employs a polyvinyl chloride (PVC) lining for the metal container. However, in this day and age, when there is great concern about possible toxic effects of materials on human beings, there has been reluctance to manufacture a tomato paste drum having a PVC lining in the United States. Indeed, such product is not sold in the United States at this time.
In contrast to the foregoing, the present invention uses an epoxy resin lining material which is applied under special conditions with unexpected and surprising results, particlularly in the light of prior art teachings of using epoxy resins and other materials. The special lining of this invention and the method of applying it to steel drum shells and lids, include coating with a special primer after cleaning the metal surfaces in a conventional manner, electrostatically spray coating with the epoxy material under specific conditions to a specified and required thickness, and baking and curing the coating at times and temperatures which are compatible with a commercial process.
There are many teachings in the prior art. However, none of them disclose a coating material of epoxy powder that is applied by electrostatic spray to a controlled specific and narrow thickness range which has been found to be essential to attain the results of the present invention. For example, several references show the use of various powders, including epoxy resins for lining pipe, coating drums or wire coating or other products where the thickness of the coat does not appear to be a problem for the user. Such uses do not have the thickness requirements of this invention and in fact rely on thicker coatings. Ruptures, cracks, chipping and uneven thin linings do not present problems to the prior art, particularly of contaminations that result when there is exposure of metal to corrosive foodstuffs. Certain of these references are discussed below.
U.S. Pat. No. 3,414,425 relates to a method for coating black steel drums with a powdered resin or metallic powder, by discharging a curtain of particulate material from a slotted opening in an elongated trough onto the inner surface of the rotating drum. The only resin disclosed, however, is polyethylene. This does not suggest the use of an epoxy and the PE resin would actually be inoperative.
The article entitled "Epoxy, Acrylic and Polyester Thermosetting powders Are Moving Toward Practical Applications" in Metal Finishing, June, 1973 Pgs 41-45, discusses epoxy, acrylic and polyester thermosetting powders and primers. This is a general review article. The only epoxy process specifically described in detailis to provide external anticorrosive protection of the Alaskan pipeline. That process is quite different than the process of the present invention.
Products Finishing, October, 1972, Pgs 98 to 100, is entitled "Lining Steel Pipe With Epoxy Powder". This article, however, describes quite different coatings, such as an interior pipe lining of 24 mils thickness.